Open interest is the total number of open or outstanding (not closed or delivered) options and/or futures contracts that exist on a given day.
BREAKING DOWN ‘Open Interest’
For every seller of a futures or option contract, there must also be a buyer of the contract. One seller and one buyer together create one contract. Therefore, the total open interest in the market for a specified futures or option market equals the total number of buyers or the total number of sellers, not the total of both added together.
Open interest of the futures and options markets is sometimes confused with trading volume, but the two terms refer to distinctly different measures. For example, on a day when one trader who already holds 10 option contracts sells those 10 option contracts to a new trader entering the market, the transfer of contracts does not create any change in the open interest figure for that particular option; no new option contracts have been added to the market. However, the sale of the 10 option contracts by the existing option holder to an option buyer does increase the trading volume figure for the day by 10 contracts. The open interest number only changes when a new buyer and seller enter the market, creating a new contract.
The Importance of Open Interest
Open interest is a measure of the flow of money into a futures or options market. Increasing open interest represents new or additional money coming into the market, while decreasing open interest indicates money flowing out of the market. An increase in open interest is typically interpreted as a bullish signal, while decreasing open interest figures are generally interpreted as a bearish sign.
Open interest is also used as a momentum indicator of trend strength. Since rising open interest represents additional money coming into a market, indicating increased interest in the market by investors, it is generally interpreted to be an indication that the existing market trend is gaining momentum and is likely to continue. Conversely, decreasing open interest reflects declining interest on the part of investors and waning momentum, indicating that the existing trend may soon be exhausted, leading to a trend change.